Frontend: simplify custom predicates interfaces (#83)

* add comments detailing logic, migrate middleware::custom::tests to frontend::custom

* simplify custom predicate's frontend interfaces, making it less verbose to define Statement Template arguments

The main idea is that when defining the arguments at a statement
template, it can be done from 3 different inputs:
i. `(&str, literal)`: this is to set a POD and a field, ie. `(POD, literal("field")`)
ii. `(&str, &str)`: this is to define a origin-key wildcard pair, ie. `(src_origin, src_dest)`
iii. `Value`: this is to define a literal value, ie. `0`

src/middleware/mod.rs

@@ -48,6 +48,12 @@ pub type Entry = (String, Value);
 #[derive(Clone, Copy, Debug, Default, Hash, PartialEq, Eq)]
 pub struct Value(pub [F; 4]);
 
+impl ToFields for Value {
+    fn to_fields(self) -> (Vec<F>, usize) {
+        (self.0.to_vec(), 4)
+    }
+}
+
 impl Value {
     pub fn to_bytes(self) -> Vec<u8> {
         self.0